Abstract
BackgroundPseudomonas aeruginosa, the rhamnolipids-producer, is one of dominant bacteria in oil reservoirs. Although P. aeruginosa strains are facultative bacteria, the anaerobic biosynthesis mechanism of rhamnolipids is unclear. Considering the oxygen scarcity within oil reservoirs, revealing the anaerobic biosynthesis mechanism of rhamnolipids are significant for improving the in-situ production of rhamnolipids in oil reservoirs to enhance oil recovery.ResultsPseudomonasaeruginosa SG anaerobically produced rhamnolipids using glycerol rather than glucose as carbon sources. Two possible hypotheses on anaerobic biosynthesis of rhamnolipids were proposed, the new anaerobic biosynthetic pathway (hypothesis 1) and the highly anaerobic expression of key genes (hypothesis 2). Knockout strain SGΔrmlB failed to anaerobically produce rhamnolipids using glycerol. Comparative transcriptomics analysis results revealed that glucose inhibited the anaerobic expression of genes rmlBDAC, fabABG, rhlABRI, rhlC and lasI. Using glycerol as carbon source, the anaerobic expression of key genes in P. aeruginosa SG was significantly up-regulated. The anaerobic biosynthetic pathway of rhamnolipids in P. aeruginosa SG were confirmed, involving the gluconeogenesis from glycerol, the biosynthesis of dTDP-l-rhamnose and β-hydroxy fatty acids, and the rhamnosyl transfer process. The engineered strain P. aeruginosa PrhlAB constructed in previous work enhanced 9.67% of oil recovery higher than the wild-type strain P. aeruginosa SG enhancing 8.33% of oil recovery.ConclusionThe highly anaerobic expression of key genes enables P. aeruginosa SG to anaerobically biosynthesize rhamnolipids. The genes, rmlBDAC, fabABG, rhlABRI, rhlC and lasI, are key genes for anaerobic biosynthesis of rhamnolipid by P. aeruginosa. Improving the anaerobic production of rhamnolipids better enhanced oil recovery in core flooding test. This study fills the gaps in the anaerobic biosynthesis mechanism of rhamnolipids. Results are significant for the metabolic engineering of P. aeruginosa to enhance anaerobic production of rhamnolipids.
Highlights
Pseudomonas aeruginosa, the rhamnolipids-producer, is one of dominant bacteria in oil reservoirs
The cell growth (OD600), surface tension and oil spreading circles diameters were shown in Fig. 1, From the biomass (OD600) point of view, strain P. aeruginosa SG grew well under anaerobic conditions using both glucose and glycerol as carbon sources (Fig. 1a)
Results indicated that strain P. aeruginosa SG produced biosurfactants when using glycerol as carbon source
Summary
Pseudomonas aeruginosa, the rhamnolipids-producer, is one of dominant bacteria in oil reservoirs. Considering the oxygen scarcity within oil reservoirs, revealing the anaerobic biosynthesis mechanism of rhamnolipids are significant for improving the in-situ production of rhamnolipids in oil reservoirs to enhance oil recovery. It is feasible to enhance oil recovery using microorganisms to in-situ produce biosurfactants in oil reservoirs [2,3,4]. It is proved that the stable microbial growth and efficient production of biosurfactants such as rhamnolipids in reservoirs are the keys to the successful implementation of MEOR [1, 3, 4]. The injection of air into oil reservoirs is difficult to ensure the effective oxygen supply of aerobic microorganisms [4]. Studies on anaerobic synthesis of biosurfactants are still weak [6], which limits to enhance oil recovery by anaerobic production of biosurfactants in oil reservoirs
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